1. Field of the Invention
The present invention generally relates to pulse width controlling logic circuits and, more particularly, to a logic circuit for automatically controlling a width of a pulse.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. 53-60550 discloses a circuit for automatically controlling a pulse width of a dial pulse. This circuit comprises a two-input comparator, an integrating circuit and a voltage averaging circuit. A dial pulse applied to an input terminal is applied to one of the two inputs of the two-input comparator via the integrating circuit. The other input of the two-input comparator is connected to an output of the voltage averaging circuit for obtaining an average of a voltage of the input pulse signal.
The integrating circuit comprises a resistor and a capacitor. Two short circuits are connected in parallel with the resistor so as to instantaneously charge and discharge the capacitor, respectively, with the input pulse signal. Each of the short circuits comprises a diode and a transistor. An output of the comparator is applied to a base of each of the transistors so as to turn the transistors on or off. With this, charging and discharging are controlled so as to obtain a desired pulse width of the input pulse signal. As a result, an input pulse signal that varies to reside outside a range of a predetermined specification is converted into a signal having a specified pulse width.
Since the circuit according to Japanese Laid-Open Patent Application No. 53-60550 requires analog circuits including the two-input comparator, the charging circuit and the discharging circuit, it has a relatively large circuit scale and is relatively expensive to manufacture as compared to a digital circuit.